Built upon the foundations of two promising operating systems (Nokia's Maemo and Intel's Moblin) it comes to market with an established feature set, a clear device strategy (MeeGo for Smartphones and MeeGo for Netbooks), a lot of early praise and two existing fan bases.
It has parallels to Windows Phone 7 by being late on the market in 2009, but a) it is fully featured (with still the best multitasking on the market), has a much publicised tablet focus and a dedicated sugar daddy in new owner HP which has promised to forgo all other mobile platforms and plough its tremendous resources into webOS development. Even Palm itself still benefits from a lot of consumer goodwill due to its numerous innovations in the past.
BlackBerry OS 6
RIM's platform already has a full feature set, but is now focused on greater usability while there are plans underway for a BlackBerry tablet. It also has a strong fan base and is dominant in the business world.
Likely failures are Vodafone 360 since it was late to market and will be shunned by rival networks and Samsung bada since it was late to market as well, is pointlessly open source given it will be shunned by rival handset makers and Samsung commits more time to Android with the likes of the Galaxy S and Galaxy Tab.
So if being years late to market and having savvy, ambitious rivals weren't enough of a mountain for Windows Phone 7 to climb the other problem is the perception of Microsoft's recent mobile track record...